Hitherto, materials having different refractive indexes and Abbe numbers have been combined to correct aberrations in the design of optical systems, such as lenses for cameras, optical disk lenses, fθ lenses, optical elements for image display media, optical films, films, various optical filters, and prisms. In order to increase variations of optical design, there is a demand for materials having various refractive indexes and Abbe numbers. Among them are materials having high refractive indexes and low Abbe numbers.
In particular, resin materials having a fluorene structure are known to have relatively high refractive indexes, low Abbe numbers, and relatively low birefringence and are expected to have high heat resistance. Thus, the syntheses of these resin materials have been studied. PTL 1 discloses a polycarbonate resin having a 9,9′-diphenylfluorene structure and having high heat resistance and mechanical strength.
Resin materials having a naphthalene structure are known to have a higher refractive index than that of resin materials having a fluorene structure. Thus, various resin materials having the naphthalene structure have also been studied. For example, PTL 2 discloses a novel aromatic diamine having a dimethylnaphthalene skeleton and having high mechanical strength, heat resistance, and processibility. The aromatic diamine can be a raw material for aromatic polyamide and polyimide resins.
However, the polycarbonate resin described in PTL 1 is produced by the homopolymerization of a monomer having a 9,9′-diphenylfluorene structure or the copolymerization of this monomer and a second monomer having a lower refractive index than the first monomer. Thus, the polycarbonate resin requires the copolymerization or the addition of a component having a higher refractive index to further increase the refractive index.
Furthermore, it is difficult to produce a polycarbonate resin useful as an optical resin easily and sufficiently for practical applications from the aromatic diamine compound having a dimethylnaphthalene skeleton described in PTL 2. It is necessary to copolymerize a dihydric alcohol having a naphthalene structure to produce a polycarbonate resin having a naphthalene structure.